In a conventional broadband local area network configuration a plurality of bidirectional amplifiers, coaxial cables, and splitters ar serially connected together to form a bidirectional rf signal path for outbound as well as inbound signal transportation. A typical conventional broadband LAN includes a plurality of such bidirectional rf signal paths. The network is designed to provide a specified path loss to each amplifier contained therein. Thus, if an outbound rf signal is connected to the network at the headend, the outbound signal will appear at each particular amplifier at a predetermined rf signal power level. Furthermore, the network is also designed to provide a specified path loss for rf signals connected to the network at the network extremities so that such inbound rf signals will appear at the amplifiers at predetermined rf signal power levels.
Temperature variations will cause the attenuation of the coaxial cable to change. This, in turn, will cause the rf signal power levels to change at the inputs to the amplifiers in the system. The effect of temperature caused variations in path loss is corrected in conventional broadband CATV type LANs by the addition of AGC circuits internal to the inbound and outbound amplifier sections of the bidirectional amplifiers in the system.
An AGC circuit comprises a closed-loop control mechanism that monitors an rf signal power level at the output of an amplifier and controls a variable attenuator so that the gain of the amplifier will automatically adjust to provide a constant rf signal power at the output of the amplifier, given a limited amount of rf signal variation at the input of the amplifier.
Conventional CATV LANs also require pilot signal generators. An outbound pilot signal generator is located at the headend of the network. This outbound pilot signal generator provides an rf signal at a certain predetermined frequency and power level that is connected at the headend and is distributed throughout the network to all outbound amplifiers. This pilot signal is monitored by the closed-loop AGC circuits and provides a measure of the path loss of the outbound signal path. Inbound paths also require pilot signal generators. The inbound pilot signal generators are located at the extremities of the network and provide an rf signal at a certain predetermined frequency and power level. Each inbound pilot signal is monitored by the closed-loop AGC circuits of only those inbound amplifiers serially connected between the injection point and the headend. Therefore, quite often it is necessary to have a plurality of inbound pilot signal generators in order to operate the entire inbound system with AGC.
Conventional CATV broadband LANs requiring multiple inbound pilot signal generators are configured in two basic types. The first utilizes multiple pilot signal generators operating at the same pilot frequency. The second utilizes multiple pilot signal generators operating at different frequencies.
Networks using multiple inbound pilot signal generators operating at the same frequency require the use of rf signal traps at intersections of the network where two inbound pilot signals combine. These traps are narrowband filters that provide a significant amount of attenuation to the pilot signal of one path so that the power level of the combined path at the pilot frequency is dominated by the power level of the untrapped pilot signal.
Multiple inbound pilot signal generators operating at different preselected frequencies do not require traps but do require frequency specific AGC circuitry for the amplifiers that are controlled by that particular frequency.
Problems with conventional CATV AGC equipment and architecture include a high degree of complexity in equipment design and manufacture, increased costs, and a high degree of complexity in network design, installation and use.
The increased degree of complexity in equipment design and manufacture includes the need to provide separate variable attenuator circuits in the inbound and outbound sections of the bidirectional amplifiers used in the system, and the need to design and manufacture inbound pilot signal generators.
The increased degree of complexity in network design includes the necessity to analyze the architecture of the system, select inbound pilot system type, either single frequency operation with traps, or multiple frequency operation with different inbound amplifier AGC circuits for each frequency specified.
The increased degree of complexity in installation includes the necessity to install inbound pilot frequency generators at the system extremities, and either the installation of traps for systems specifying a single inbound pilot frequency, or the installation of different types of inbound AGC circuits in systems specifying multiple inbound pilot frequencies. In addition, once the equipment has been installed, it is necessary to adjust the inbound pilot signal generators to the proper output level and then proceed to adjust all inbound AGC circuits to provide proper operation of the system.
The increased degree of complexity in use of a conventional LAN system includes the need for routine maintenance of the inbound pilot signal generators and the inbound AGC circuits. In addition, in systems utilizing only one inbound pilot frequency with traps, the pilot signal that is trapped cannot be monitored at the headend. Therefore AGC problems in that portion of the inbound system comprising the equipment operating from the trapped pilot cannot easily be determined at the headend.
Accordingly, it is the object of the invention to provide LANs having automatic gain control that are easier to design, manufacture, install and maintain than prior art LANs. More particularly, it is an object of the present invention to provide a LAN which requires no inbound pilot signals, no inbound pilot signal generators, no inbound automatic gain control circuits and no traps. It is a further object of the present invention to provide a LAN which achieves automatic gain control in both the inbound and outbound directions through use of an outbound pilot signal only and outbound automatic gain control circuitry.